1. Field of the Invention
This invention relates to packaging of semiconductor integrated circuit (IC) chips, and more particularly to a method for manufacturing flip chip package devices to improve the heat dissipation of the package.
2. Description of the Related Art
Flip chip technology provides a method for connecting an IC device to a substrate (e.g., a printed circuit board). To provide a flip chip package, a number of round or hemispherical solder connectors are formed on corresponding electrode pads which reside on the active surface of an IC device. Next, with the active surface facing the substrate, the IC device is mounted on the substrate by reflow-bonding the solder connectors of the IC chip to terminal pads formed on the substrate. During the bonding, heat is applied to reflow the solder connectors in order to form an electrical and mechanical connection between the substrate and the active surface of the IC device. Since the flip chip package does not employ bonding wires, the length of the electrical connection is shorter, and the dimension of the package can be reduced to as small as the chip size.
However, thermal problems do exist with the flip chip package. This is due mainly to thermal coefficient of expansion (TCE) mismatch between the IC device and the substrate. Particularly, in high power IC chips, such as a high-speed CPU chip, a design to effectively dissipate heat generated by the IC device is of great importance. Generally, to compensate for the heat, the high power flip chip device includes a heat spreader or a heat sink attached to the backside (non-active surface) of the IC chip via a Thermal Interface Material (TIM).
These conventional flip chip devices are assembled or packaged by the following process. Solder connectors are formed on the electrode pads which reside on the active surface of an IC chip. A substrate having conductive pads corresponding to the electrode pads is prepared. Next, the IC chip is connected to the substrate by connecting the solder connectors of the IC chip to the pads on the substrate and reflow-soldering the solder connectors. Before the reflow, a flux may be transferred to the solder connectors by using a dipping technique or dispensing technique. The flux may also be applied to the pads on the substrate before the reflow. A conventional underfill is then applied to fill the space between the active surface of the IC chip and the substrate using capillary force. Following this, a solder TIM is applied or attached to the backside of the IC chip and a heat spreader is connected to the IC chip.
The connection temperature of the heat spreader depends on the composition of the solder TIM. When the solder TIM comprises both of solder and flux, the connection temperature is lower, while the temperature increases for a solder TIM having only solder. Conventionally, the temperature for connecting the heat spreader to an IC chip ranges from about 280° C. to 340° C.
If the connecting temperature is high, the property of the underfill material bonding the IC chip to the substrate is degraded and the reliability of the substrate is decreased. Conventional organic substrates such as printed circuit boards suffer degradation at temperature above 250° C. When the connecting temperatures is lower however, a void may form at the interface of the solder TIM with the heat spreader. This results in the degradation of the heat dissipation.
Moreover, when the heat spreader is attached to the IC chip and is hermetically bonded to the substrate, a void may form in the hermetic sealant, which leads to failure during a reliability test such as PCT (Pressure Cooker Test) performed under a hot and humid environment (since the void provides a path along which water can penetrate).